Add flash memory support

[TheDeepestSpace]

Research and design flash memory support for our ASIC solution. We want to be able to read an image from the flash on boot.

Things to ponder:

• what will the initialization procedure look like?
• will we be able to store/run larger programs (e.g. larger than on-chip RAM) with some hardware controller orchestrating the memories?
• will we be able to keep upstream core unaware of the harsh reality of slower flash, i.e. will we be able to expose a single-clock-cycle memory to it at all times?

[TheDeepestSpace]

I will probably want this to be moved into the main repo/or have a separate repo for peripherals in general. Especially since we are no longer targeting just SKY130, would want this to be more generic. I'll set up a separate issue for this in the upstream repo, then close this.

[TheDeepestSpace]

Let's cook up a plan for this first, especially as to how we are going to test it both simulation-wise and physically. Will probably want to reach out to Devboards team to make sure they put a flash on their board.

[TheDeepestSpace]

@robertsaabwoo feel free to post your SPI implementation if you want